Method For Metal Recovery and Leaching Agent Recycle in Agitation Leach Plants

ABSTRACT

This invention is directed to an improved process for metal recovery from ore using agitation leaching comprising dividing leaching of the crushed and mined ore into at least two sequential leaching-solids-liquid separation-solvent extraction sub-circuits, with no significant dilution during solids-liquid separation in these units, and the raffinate from solvent extraction being recycled back to the leaching, with the underflow pulp from the second liquid-solids separator being sent to a final solid-liquid separator, with water washing, from which the washed solids are sent to disposal and the clarified aqueous wash solution is sent to a final solvent extraction, with some or all of the metal-depleted aqueous raffinate from this final solvent extraction being optionally neutralized, and/or being circulated back to the third solid-liquid separation as wash solution and/or to recovery of other metals and/or to disposal to maximize metal recovery and maintain water balance.

FIELD OF THE INVENTION

The present invention relates to the design and operation of theleaching and solvent extraction steps in a metal recovery plant forrecovering desired metal values from mined ores containing such metalvalues possibly comingled with other metal values.

BACKGROUND OF THE INVENTION

To obtain metals (e.g., copper, nickel, cobalt, zinc, uranium, and thelike) in a pure, useful form, these metals must be removed and recoveredfrom the ores in which they are found through a series of physical,hydrometallurgical and/or chemical steps.

Conventionally, the mined ore, containing a greater or lesser amount ofthe desired metal value, in addition to possibly one or more othermore-or-less desirable metal values and a large amount of gangue andother more-or-less complicating minerals, is leached with an aqueousacidic (commonly sulphuric acid) or basic (commonly ammonium hydroxide)solution. This leaching is accomplished by either distributing theleaching agent over a pile or bed of mostly dry ore solids in dumpleaching, heap leaching or vat leaching, wherein these ores are eitherleached as mined, or they may be crushed, but not ground or milled, to asize that gives higher metal recovery and/or faster metal recovery, or,as in agitation leaching, by mixing the leaching agent with an aqueousslurry of crushed and milled ore solids in one or more stirred tanks inan attempt to ensure optimal distribution of the leaching solutionthroughout the ore solids.

In heap, dump or vat leaching, dry ore is placed in a pile/bed leachsystem, or, where ore is agglomerated with moisture prior to beingplaced in a bed/heap, with only a small amount of added water. Withthese methods, there is often significant evaporation of any water fromthe pile/bed, and, so as not to depend solely on such evaporation tokeep the ore dry, most plants using pile/bed leach systems employ atleast one, and often several, large ponds in which to hold water thatmay accumulate in a short event, such as a heavy rain. Thus, there is noneed to bleed water on a continual basis from a heap/dump/vat leachsystem.

By comparison, in a plant employing agitation leaching, crushed ore thatis to be agitation-leached is generally ground or wet-milled to adesired size distribution for achieving an acceptable metal recovery inleaching, with the resulting ore solids being added to the agitationleach unit(s) as aqueous slurry. Thus, in agitation leaching, aconsiderable amount of water is normally brought into the leachingsystem with the ore. This water must eventually leave or be removed fromthe system in order to maintain a water balance and it does so, mainlyand continually, with the leached solids in the tailings or byintermittent bleeds from the circuit. Any desired metal or othervaluable metals in the water leaving with the leached solids is lost(called the “soluble metal loss”). In addition, any leaching agent inthis water is also lost and often has to be neutralized prior to thefinal disposal of the leached solids.

Selection of the type of leaching to be employed is based on severalfactors including the grade of the ore, the clay content of the ore, thehardness of the ore and the way the ore responds to the various leachingmethods. A dump or heap leach system is generally much less costly inboth capital (equipment) costs and operating (energy) expense, and istherefore selected for use with lower grade ores, where costs arecritical, or with higher grade ores that respond well to heap leaching,permitting a high metal recovery. Agitation leaching, on the other hand,provides for a faster and more complete recovery of the desiredmetal(s), is easier to control, and often gives higher recovery ofsecondary valuable metals, such as cobalt, but it is also more expensivedue to the capital cost of additional equipment, such as mills, leachtanks and clarifiers, and has a higher operating cost because of, forexample, the energy required to mill the ore and the chemicals neededfor the solids-liquid separation.

Following the leaching step in a circuit employing agitation leaching(such circuits being the focus of this invention), the resulting mix ofaqueous leachate, now containing a high proportion of the desired metalvalues, as well as leached ore solids from which the desired metalvalues have been dissolved, is then normally sent to a solids-liquidseparation process, such as by counter-current decantation (“CCD”), withwashing, or by filtration, also with washing. Following thissolids-liquid separation process, the clarified or partially-clarifiedaqueous phase is sent to one or more units in a solvent extractionprocess for transfer of the metal values from the aqueous leachate intoan organic phase comprising one or more extraction reagents.

In that solvent extraction process, the particular desired metal valueis extracted from the leach solution containing that metal value into anorganic phase by one or more extraction reagents specific for thatdesired metal, which reagent(s) is/are dissolved in an organic phasethat comprises the extraction reagent(s), optionally with one or moreequilibrium modifiers, kinetic additive(s) and/or other compounds, in awater-insoluble, water-immiscible organic solvent. During suchextraction, hydrogen ions are released from the organic phase into theaqueous phase, now largely depleted of the desired metal values, asrepresented by the equation below for extraction when copper is thedesired metal, sulphuric acid is the leaching agent, and where “RH”represents the copper-specific extraction reagent(s):

2RH+CuSO₄⇄R₂Cu+H₂SO₄

In the extraction of 1 ton of copper, 1.54 tons of sulphuric acid(useful for further leaching of copper when the leach solution, depletedof copper values, is returned to the leaching unit(s)) is regenerated inthe leach solution from which the copper was extracted. Thus, thegreater the amount of copper extracted from an aqueous solution, thehigher the concentration of sulphuric acid generated in that solution.In general, more of the leached copper can be extracted when theconcentration of copper in the leach solution is higher, thus, thehigher the copper concentration in the leach solution to be treated bysolvent extraction, the greater is the potential to return moresulphuric acid back to the leaching unit. Other metals, such as Zn, Niand Co, also show this behavior, depending on the leach solution andextraction reagent(s) employed.

Following the extraction, the metal-rich organic phase containing one ormore complexes of the desired metal with the extraction reagent(s) isthen possibly washed to reduce the level of undesired iron and/or otherundesirable species, and stripped of its desired metal content with astripping agent, such as a relatively concentrated acid solution(normally sulphuric acid) that breaks apart the complex(es), freeing thedesired metal into the aqueous “pregnant stripping solution”. That metalis then finally captured in a pure form from the desired metal-richpregnant stripping solution, by electrodeposition in an electrowinningstage, or by one or more alternative metal recovery processes.

The great quantities of solids and the large volumes of leaching andstripping agents, extraction reagents, organic solvents and purifiedwater involved in large-scale mining and metal recovery operationsmandate efforts to use these resources most efficiently, both from apurely economic perspective, and in consideration of the potentialenvironmental impact of accidental discharges and intentional disposalof no-longer-useful substances. Increased recycling of expensive agentsand reagents, and the reduction of losses resulting both from thedisposal of metal-depleted tailings slurry still containing some desiredmetal and other metal values dissolved in the water in the tailingsslurry and from bleeds of the aqueous phase in order to maintain theoverall water balance and/or adjust/correct levels of undesirable metalsor acid, as well as from other conservation measures, have becomecritical to the successful economic and environmentally-responsibleoperation of mining and metal recovery operations.

A particular issue addressed by this invention is the high cost both ofreplacing leaching agents lost or bled from the leaching-solventextraction-electrowinning circuits and of purchasing substances used toneutralize excess leaching agents prior to further metal recoveryactivities and/or disposal of spent leaching and/or washing solutionscontaining these leaching agents. Another issue addressed is the need torecover a higher percentage of the desired metal values from the leachedores and thereby reduce the amount of valuable metal that is ultimatelylost from the circuit in bleed streams or to tailings disposal,resulting in the loss of significant revenue that could be realized bythe operator.

US 2005/0031512 A1 (Kordosky et al) showed that good metal extractionmay be achieved while also significantly improving the recovery of theleaching agents by proposing a “splitcircuit” arrangement of leachatesolution flows that does not require as much fresh leaching agent to beadded to supplement the recycling of the leachate solution back to theleaching stage. This method does not follow the conventional practice ofwashing, and thereby diluting, the entire solution flow from the one ormore agitation leach units during the solids-liquid separation stagefollowing leaching. Such washing is intended to minimize the loss ofmetal values with the disposal of the metal-depleted tailings slurry,but it also reduces the concentration of the desired metal in theclarified leach solution exiting solids-liquid separation and therebyreduces the leaching agent concentration which can build in thissolution as the desired metal is extracted. Since only a portion of thisleach solution, now depleted of desired metal values, but increased inleaching agent concentration, is recycled back to leaching, lessleaching agent is recycled back to leaching than would be if theleaching solution had not been diluted.

Instead, the split circuit design involves subjecting the first leachedpulp from the leach unit(s), comprising a mixture of metal-depletedleached solids and an aqueous leach solution containing dissolved saltsof the desired metal, leaching agent, water, and possibly other metalvalues, to a first solids-liquid separation, without significantdilution. The solids pulp from that separation is then sent to a secondsolids-liquid separation, with significant washing/dilution, with theclarified metal-rich aqueous leach solutions from each solids-liquidbeing separation circulated to separate solvent extraction units. Thesolids, as aqueous slurry, from the second solids-liquid separation arethen sent to disposal, with the metal-depleted raffinate from thesolvent extraction unit(s) following the first solids-liquid separation,without dilution, being recycled as leach solution, possiblysupplemented with additional fresh leaching agent, to one or more of theleach unit(s). The raffinate, depleted of desired metal values, exitingthe solvent extraction unit(s) following the second solids-liquidseparation is neutralized, as necessary, and/or circulated to one ormore additional units to possibly recover other metal values that mayalso have been present in the original ore in sufficient quantities,and/or recycled back to the second solids-liquid separation as washsolution, with the possibility that some of the neutralized solution maybe bled to disposal in order to maintain a water balance.

Nevertheless, mineral industry is still interested in improved processesfor the leaching of ores allowing significant reductions in leachingagent replacements and neutralizations, as well as further significantincreases in the recovery of the desired metal from the original ore.The problem underlying the present invention has been to serve theseneeds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a process for leaching desiredmetal values from crushed and milled ore solids and extracting thosevalues into organic phases for further recovery efforts, in order toeventually obtain the desired metal in a usable form. This processinitially comprises leaching crushed and milled ore solids with anacidic or basic leaching solution in one or more initial/“first”agitation leach units, to dissolve a significant portion of the desiredmetal values from the crushed and milled ore solids into an aqueousphase. The slurry of partially-leached solids with aqueous leachsolution (called a “leach pulp”) resulting from the initial leachingunit(s) proceeds to a first solids-liquid separation and clarificationto produce two products, a first undiluted aqueous leach solution, richin desired metal values, and a second leach pulp.

The first undiluted aqueous leach solution is then circulated, withoutsignificant dilution, to one or more “first” solvent extraction unitsfor extracting the desired metal values from the aqueous solution intoan organic phase. From the first solvent extraction unit(s), the aqueoussolution (“raffinate”), depleted of desired metal values, is recycled asleaching solution, possibly augmented by fresh leaching agent and/orrecycled raffinate from one or more other solvent extractions later inthe process/circuit, back to the initial leach unit(s).

After the first solid-liquid separation and clarification, the secondleach pulp is sent, without significant dilution, to one or more “final”agitation leach units for additional leaching, in order to dissolveanother significant portion of the desired metal values remaining in thepulp. The leach pulp resulting from the “final” leach unit(s) issubjected to a second solids-liquid separation and clarification toproduce two products, a second undiluted aqueous leach solution, rich indesired metal values, and a third leach pulp. The second undilutedaqueous leach solution is circulated, without significant dilution, toone or more “second” solvent extraction units for extracting the desiredmetal values from the aqueous solution into an organic phase, with theaqueous solution (“raffinate”), depleted of desired metal values,exiting the second solvent extraction unit(s) being recycled as leachingsolution, possibly augmented by fresh leaching agent and/or recycledraffinate from one or more other solvent extractions earlier and/orlater in the process/circuit, back to the final agitation leach unit(s).

After the second solids-liquid separation and clarification, the thirdleach pulp, now largely depleted of desired metal values after twoleachings, is sent to a last solids-liquid separation, with waterwashing and significant dilution for the first time, from which thewashed solids slurry is sent to disposal, and the clarified aqueous washsolution is sent to one or more final solvent extraction units. From thefinal solvent extraction unit(s), the aqueous solution (raffinate),depleted of desired metal values, is optionally neutralized and theneither possibly sent to one of more units to recover any other valuablemetal values also present in the original ore, or recycled back to thelast solid-liquid separation unit(s) as wash solution, or it may besplit, with some portion sent to recovery of other metal values, andsome portion sent to recycle back to the last solids-liquid separationunit(s) as wash solution, and perhaps even some to final disposal.

In some cases, depending on the grade of ore and the leachingcharacteristics of that ore, there may be one or more additional“intermediate” sub-circuits of leaching/solids-liquid separation/solventextraction steps similar to the initial or first leaching/solids-liquidseparation/solvent extraction sub-circuit, inserted between the firstand “final” leaching/solids-liquid separation/solvent extractionsub-circuit.

By following this process and changing the current design and operationof the leaching and solvent extraction processes of currentmetal-recovery plants accordingly, the amount of leaching agent recycledto leach may be significantly increased, and both the amount of leachedmetal that is lost to tailings disposal and the quantities of additionalfresh leaching agent purchased and the quantities of chemicals that mustbe expended to neutralize excess leaching agent in the circuits may besignificantly decreased.

Surprisingly it has been observed that It surprisingly has now beenfound that further significant reductions in leaching agent replacementsand neutralizations, as well as further significant increases in therecovery of the desired metal from the original ore, may be realizedover the “split circuit” arrangement by dividing the duty of leachingthe desired metal values from the original crushed and milled ore solidsamong two or more agitation leach units in series. Each of these unitsthen leaches desired metal values from the same ore solids, with eachunit being followed by its own solids-liquid separator, withoutsignificant dilution, then its own solvent extraction unit(s), prior toa final solids-liquid separation, with washing, and a final solventextraction on the clarified solution exiting the final solids-liquidseparation, with washing, to try to recover any final amounts ofvaluable metal. The metal-depleted aqueous solution exiting the finalsolvent extraction unit is neutralized, as necessary, and/or circulatedto one or more additional units to possibly recover other metal valuesthat may also be present in the original ore, prior to disposal and/orrecycle back to the final solid-liquid separation, with washing, as washsolution. The metal-depleted aqueous slurry of the leached solidsexiting the final solids-liquid separation, with washing, is then sentto final disposal, which, in most cases, includes neutralization. Withthis new circuit design, raffinates from each solvent extraction unit,except the final solvent extraction unit, may be totally recycled to oneor more of the preceding agitation leach unit(s), and, in doing so, muchmore leaching agent is recycled to leaching and, therefore, much lessleaching agent is lost to final disposal as compared to the conventionaland “split circuit” flow sheets. In addition, the desired metal lost tofinal disposal in the leached and washed solids is minimized whencompared to either the conventional or “split circuit” flow sheets.

In particular, it has been found that by not depending on a singleinitial leach unit to dissolve all or mostly all of the desired metalvalues from the crushed and milled ore solids at one time, and breakingthe leaching function into two or more units in a series or sequentialarrangement, with accompanying solids-liquid separators, withoutdilution, and solvent extraction units, as described below, the amountof desired metal that may be recovered and the amount of leaching agentthat may be recycled may be substantially increased. According to thisinvention, the crushed and milled ore solids are subjected to a sequenceof leach units, each leach unit dissolving a portion of the desiredmetal values from, effectively, the same crushed ore solids (theoriginal crushed and milled ore solids in the first agitation leach unitand progressively-more-leached solids pulps in subsequent agitationleach units in the series) with each such leaching unit being followedby its own solids-liquid separation without significant dilution, thenone or more solvent extraction units to extract the desired metal valuefrom the aqueous leach solution, rich in desired metal values, comingfrom the respective solids-liquid separation unit(s). In addition, all,or almost all, of the aqueous raffinates regenerated by the solventextractions are recycled back to either their respective leach units, orrecycled among two or more of the previous or following leach units inthe circuit, for additional leaching, prior to a final solids-liquidseparation, with washing. That final separation is applied to thecrushed and milled leached solids exiting the last solids-liquidseparation without significant dilution, and is followed by a finalsolvent extraction on the clarified leach solution exiting the finalsolids-liquid separation.

To understand the significant benefits of the present invention over theconventional standard agitation leaching-solvent extraction operation,as well as over the “Split Circuit” agitation leaching-solventextraction process, and not depend on any particular theory, involvesclosely comparing the flow diagrams and accompanying mass balances inFIGS. 1 through 3, with copper as the desired metal and sulphuric acidas the leaching agent, as later explained in the Examples.

For example, in any copper agitation leach-solvent extraction recoveryprocess, all the sulphuric acid recycled back to leaching may be used toleach more copper, while all the acid taken to neutralization orcontained in the tailings is lost, and, therefore, cannot be used toleach more copper. The more acid that can be recycled, the less acidthat needs to be purchased, and the less the amount of acid that must beneutralized and/or that would be lost to disposal.

In each flow sheet, copper recovery from leaching is set at a realistic90% and copper recovery from solvent extraction is also assumed to be arealistic 90%, even though copper recovery in an agitation leachingprocess can be up to nearly 100% in some cases and copper recoveryacross a copper solvent extraction unit can be more than 90% in somecases.

In one aspect, the instant invention provides a process for recoveringmetal values from crushed and milled ore solids comprising desired metalvalues that may be comingled with one or more other metal values, whichprocess comprises:

-   -   (a) mixing a first aqueous leach solution with a body of the        crushed and milled ore solids in a first agitated tank leach        unit in order to dissolve at least a significant portion of the        desired metal values formerly in the ore solids into the first        aqueous leach solution and to obtain a first aqueous leach pulp,        which pulp comprises a mixture of leached solids and first        aqueous leach solution, rich in the desired metal values;    -   (b) subjecting the first aqueous leach pulp to a first        solids-liquid separation, without significant water dilution, to        provide a first clarified aqueous leach solution and a second        aqueous leach pulp, which pulp comprises leached solids at a        percent solids level that is greater than that in the first        aqueous leach pulp;    -   (c) sending the second aqueous leach pulp to a final agitated        tank leach unit, and circulating the first clarified aqueous        leach solution to a first solvent extraction, wherein, in such        solvent extraction, at least a significant portion of the        desired metal values are extracted into a first organic phase by        one or more extraction reagent(s) specific for the desired        metal, which extraction reagent(s) is/are dissolved in an        organic formulation that comprises such extraction reagent(s),        optionally with one or more equilibrium modifiers, kinetic        additives and/or other compounds in a water-insoluble,        water-immiscible organic solvent, creating a first organic        phase, rich in the desired metal as one or more desired        metal-extraction reagent(s) complexes, that is sent to further        metal recovery processes, and a first aqueous raffinate,        depleted of desired metal values, up to all of which raffinate        may be recycled/circulated back to the first agitated tank leach        unit as at least a part of the first aqueous leach solution,        which solution may be supplemented by fresh leaching agent        and/or one or more other raffinates from later in the process;    -   (d) mixing a second aqueous leach solution with the second        aqueous leach pulp in the final agitated tank leach unit in        order to dissolve another portion of the desired metal values        formerly in the partially leached crushed and milled ore solids        (now comprising the second aqueous leach pulp) into the second        aqueous leach solution and to obtain a third aqueous leach pulp,        which pulp comprises a mixture of twice-leached solids and a        second aqueous leach solution, rich in desired metal values;    -   (e) subjecting the third aqueous leach pulp to a second        solids-liquid separation, without significant water dilution, to        provide a second clarified aqueous leach solution and a fourth        aqueous leach pulp, which pulp comprises leached solids at a        percent solids level that is greater than that in the third        aqueous leach pulp;    -   (f) sending the fourth aqueous leach pulp to a third        solids-liquid separation, and circulating the second clarified        aqueous leach solution to a second solvent extraction, wherein,        in such solvent extraction, at least a significant portion of        the desired metal values are extracted into a second organic        phase by one or more extraction reagent(s) specific for the        desired metal, which extraction reagent(s) is/are dissolved in        an organic formulation that comprises such extraction        reagent(s), optionally with one or more equilibrium modifiers,        kinetic additives and/or other compounds in a water-insoluble,        water-immiscible organic solvent, creating a second organic        phase, rich in the desired metal as one or more desired        metal-extraction reagent(s) complexes, that is sent to further        metal recovery processes, and a second aqueous raffinate,        depleted of desired metal values, up to all of which raffinate        may be recycled/circulated back to the final agitated tank leach        unit as at least a part of the second aqueous leach solution,        which solution may be supplemented by fresh leaching agent        and/or one or more other raffinates from earlier or later in the        process;    -   (g) subjecting the fourth aqueous leach pulp to a third        solids-liquid separation, with significant dilution via an        aqueous stream, in order to obtain a third clarified aqueous        leach solution, wherein the concentration of desired metal        values in the third clarified aqueous leach solution is less        than the concentration of desired metal values in the second        clarified aqueous leach solution, and a fifth aqueous pulp,        which pulp comprises a mixture of leached solids and aqueous        leach solution; and    -   (h) sending the fifth aqueous pulp to disposal and circulating        the third clarified aqueous leach solution to a third solvent        extraction unit, wherein, in such solvent extraction, a third        organic phase of water-insoluble, water-immiscible organic        solvent formulation comprising one or more extraction reagents        extract at least a portion of the desired metal values from the        third clarified aqueous leach solution creating a third organic        phase, rich in the desired metal as one or more desired        metal-extraction reagent(s) complex(es), that is sent to further        metal recovery processes, and a third aqueous raffinate,        depleted of desired metal values, that is optionally neutralized        and circulated back to the third solids-liquid separation as at        least a part of the aqueous washing solution to recover at least        a portion of any remaining desired metal values from the fifth        aqueous pulp, or is optionally neutralized and sent to disposal,        or is optionally neutralized and treated to recover one or more        other metal values, if present in sufficient amounts, that may        be present in the mined ore solids, or is optionally neutralized        with portions circulating back to the third solid-liquid        separation and/or to further metal recovery and/or to disposal.

For purposes of clarity, in each instance in this process, whenreference is made to a single “unit”, it should be understood that such“unit” may actually be several units in parallel or in series.Specifically, each leaching “unit” may consist of several agitatedleaching tanks in parallel or in series, and each solvent extraction“unit” may consist of a single stages or a multiple number of stages,either extraction only or extraction and stripping in a typicalarrangement, such as solvent extraction units or stages in parallel orseries. It is also possible that all of the solvent extraction units areactually just different stages in a single solvent extraction plant.Generally, the solvent extraction process is highly flexible and theparticular arrangement of solvent extraction units or stages for anygiven leach solution is done in order to optimize recovery of thedesired metal and to optimize regeneration of the leaching agent forrecycle.

In the present inventive process, the leached-solids pulp from eachsolids-liquid separator, prior to the final one that does involve afinal washing/dilution, becomes the leachable body of the followingleach unit (the leach unit next in the series/sequence). It should alsobe understood that each solids-liquid separation, with or withoutdilution/washing, may be conducted in any manner capable of separatingsolids from liquids; the method of such separations is not critical. Forexample, solids may be separated from liquids by methods including, butnot limited to, decantation and/or filtration. In the finalsolids-liquid separation, with significant washing/dilution, accordingto the invention, counter-current decantation is preferred, but is notmandatory.

The term, “significant dilution” or “significant washing/dilution”, whenused in the process in accordance with the instant invention, refers tothe addition of a measurable amount of water or other aqueous solution.Dilution of any of the clarified leach solutions prior to circulation ofthem to solvent extraction could cause a build-up of the volume ofaqueous phase in one of the loops, and as such, would be undesirable andcould decrease leaching agent recovery. Significant dilution of suchaqueous leach solution is only used in the instant process in the finalsolids-liquid separation as part of the final solids—liquid separationwash process to try to recover the last vestiges of the desired metalvalues from the pulp prior to disposal of the metal-depleted ore solids.

Additionally, the solvent extractions in accordance with the processesof the present invention may also be carried out in any known manner,wherein aqueous leach solution is contacted with an organic phasecontaining an extraction reagent, specific to the desired metal. Forexample, these solvent extractions may be carried out usingmixer—settler solvent extraction units, wherein the organic phase andthe aqueous leach solution are vigorously intermixed in a mixer, and theresulting dispersion of organic and aqueous is then passed to a settlerwhere the two phases settle, and from which there exits a clear organicphase and a clear aqueous phase.

Also, the “further metal recovery processes” to which the organicphases, rich in the desired metal values, may be subjected mightcomprise additional metal extraction followed by washing with a solutiondesigned to remove undesirable species prior to contacting the organicphase, rich in desired metal values, with a suitable stripping agentthat breaks apart the desired metal-extraction reagent complex andallows passage of the desired metal into an aqueous phase containing thedesired metal in a concentrated and purified state from which finalmetal recovery takes place by electrowinning, or one or more other finalmetal recovery methods. With certain metals is may also be possible torecover the desired metal directly from the organic phase, rich indesired metal values, even though this is not a common technique.

And all solutions, phases, raffinates, and pulps may be conveyed withinthe circuits of the process by pipes or any other natural or man-madeconduit.

The process according to the instant invention may be practiced in a newplant designed specifically for the instant invention, or it may bepracticed in an existing plant by reconfiguring existing equipment, andpulp and solution flows, without necessarily adding a great deal ofhandling and/or process equipment.

In a preferred application of the process according to the instantinvention, a majority of the desired metal values in the mined ore isintended to be leached from the crushed and milled ore solids in aninitial leach unit, at least a majority of the desired metal valuesremaining in the solids pulp from the solids-liquid separator followingthe initial leach unit is leached in the next leach unit, and, inembodiments of the instant process comprising more than two leach units,the number of such units being limited by the economics of diminishingreturns, the desired metal values in the mined ore are leached insequential leach units progressively from a majority in the initialleach unit in the total circuit, a majority of the metal valuesremaining being leached in the next leach unit, and so on, until thelast reasonably-recoverable amount of the remaining desired metal valuesfrom the original crushed and milled ore solids are leached by the finalleach unit in the process/circuit.

For example, in a preferred application of the instant process, whichcomprises leaching with two leach units, 60 to 75% of the desired metalvalues in the original ore might be preferably leached in the firstleach unit, and the remaining 25 to 40% of such desired metal valueswould then be leached in the second leach unit. In a preferredapplication of the instant process, which comprises leaching the desiredmetal values with three leach units, 45 to 55% of such desired metalvalues in the original ore might be preferably leached in the firstleach unit, 25 to 35% of such desired metal values might be preferablyleached in the second leach unit, and the remaining 10-30% of suchdesired metal values might preferably be leached in the third leachunit. In a particularly preferred aspect of the process according to theinstant invention, at least a majority of the original or remainingdesired metal values from the ore solids or pulp, as the case may be, isremoved in the initial and each successive leach unit-solvent extractionunit combination, in order to maximize the regeneration of the leachingagent during the process and thereby maximize the recycling of suchleaching agent to the leaching process. This sequential leachingpractice generally results in the concentration of desired metal in thefirst clarified aqueous leach solution being at least 30% greater thanthe concentration of the desired metal in the second clarified aqueousleach solution, preferably this difference is at least 50%, morepreferably this difference is at least 70%, and still more preferablythis difference is 100%.

In another embodiment of the instant invention, one or more intermediateagitation tank leach units may be inserted after the first solid-liquidseparation and before the final agitated tank leach unit in step (c),such intermediate agitated tank leach unit sending an aqueous leachpulp, resulting from an aqueous leach solution being distributed throughan aqueous leach pulp coming from the first solids-liquid separation, toan intermediate solids-liquid separation, from which an intermediateaqueous leach pulp is sent to the final agitated tank leach unit, and anintermediate clarified aqueous leach solution is circulated to anintermediate solvent extraction, from which an intermediate aqueousraffinate up to all of which raffinate may be recycled/circulated backto the intermediate agitated tank leach unit as at least a part of theaqueous leach solution for such leach unit, which solution may besupplemented by fresh leaching agent, and an intermediate desiredmetal-rich organic phase, rich in desired metal values, is sent tofurther metal recovery processes. It being readily understood by theskilled practitioner that the number of additional such sub-circuitswould be determined by economic practicality, i.e., the capital andoperating cost of the sub-circuit will be measured against thediminishing returns that may be realized in further recovery of desiredmetal and reduction in leaching agent and neutralization substances.

For purposes of illustration, one such additional sub-circuit maycomprise a leach unit, labelled an “intermediate agitation leach unit”,a solids-liquid separator labelled an “intermediate solids-liquidseparation”, following this agitation leach unit, and a solventextraction following the intermediate solids-liquid separation, thissolvent extraction labelled an “intermediate solvent extraction”. Suchan intermediate agitation leach unit might be inserted after the firstsolids-liquid separation and before the final agitated tank leach unitin step (c) in the process described above, such intermediate agitationleach unit sending an aqueous leach pulp, resulting from an aqueousleach solution being distributed through an aqueous leach pulp comingfrom the first solids-liquid separation, to an intermediatesolids-liquid separation, from which an intermediate aqueous leach pulpis sent to the next agitation leach unit, and an intermediate clarifiedaqueous leach solution is circulated to an intermediate solventextraction, from which exits an intermediate aqueous raffinate, up toall of which raffinate may be recycled/circulated back to theintermediate leach unit, or an earlier or later agitation leach unit, asat least a part of the aqueous leach solution for such leach unit, whichsolution may be supplemented by fresh leaching agent, and anintermediate organic phase, rich in the desired metal as a desiredmetal-extraction reagent(s) complex(es), is sent to further metalrecovery processing.

The process according to the instant invention may be used in any metalrecovery operation which employs an aqueous agitation leachingoperation, where the leaching agent is regenerated in the solventextraction process, and essentially with any leaching agent that iswater-miscible, capable of leaching the desired metal from the mined oreinto the desired metal leaching solution. Such leaching agents include,but are not limited to acids, including sulphuric acid, hydrochloricacid, nitric acid, organic acids, and combinations of two or morethereof, and basic substances, including gaseous ammonia and ammoniumhydroxide. In certain preferred embodiments of the present invention,the leaching agent is sulphuric acid, resulting in each aqueous leachsolution, i.e., the first aqueous leach solution, the second aqueousleach solution, the third aqueous solution, any intermediate aqueousleach solution, and so on, as well as each raffinate, i.e., the firstaqueous raffinate, the second aqueous raffinate, the third aqueousraffinate, and so on, being sulphuric acid solutions. In other certainpreferred embodiments of the instant invention, the preferred leachingagent is gaseous ammonia or ammonium hydroxide, resulting in each of theleach solutions, i.e., the first aqueous leach solution, the secondaqueous leach solution, the third aqueous solution, any intermediateaqueous leach solution, and so on, as well as each raffinate, i.e., thefirst aqueous raffinate, the second aqueous raffinate, the third aqueousraffinate, and so on, being ammonia/ammonium hydroxide solutions.

The process of the invention is preferably used in the leaching andsolvent extraction of desired metals that occur naturally as oxideand/or sulphide ores, preferably in the leaching and solvent extractionof divalent metals, such as copper, zinc, nickel and cobalt, andincluding, for example, transition metals. In a preferred embodiment ofthe invention, the desired metal is copper, and, particularly, when thedesired metal is copper, the preferred leaching agent is sulphuric acid.In another preferred embodiment of the invention, the desired metal iscopper, and the preferred leaching agent is gaseous ammonia or ammoniumhydroxide. In still another preferred embodiment, the desired metal iszinc, and particularly, when the desired metal is zinc, the leachingagent is sulphuric acid or gaseous ammonia or ammonium hydroxide. In yetanother preferred embodiment of the invention, the desired metal isnickel, and, particularly, when the desired metal is nickel, thepreferred leaching agent is sulphuric acid or gaseous ammonia orammonium hydroxide. In another preferred embodiment of the invention,the desired metal is cobalt and the preferred leaching agent issulphuric acid.

The aqueous raffinate from each solvent extraction process is generallyrecycled back to the leach unit from which the clarified aqueous leachsolution that was circulated to that vessel originated most recently inorder to leach more desired metal from the crushed and milled ore solidsor a subsequent leach pulp. However, a portion of the first aqueousraffinate, a portion of the second aqueous raffinate, a portion of thethird aqueous raffinate, a portion of any intermediate aqueousraffinate(s), or a mixture of two or more thereof, may be circulated toany of the leach units and/or the third solids-liquid separator in theprocess according to the present invention if needed to maintain a waterbalance or to more efficiently distribute leaching agent.

The first aqueous raffinate produced in accordance with the processes ofthe present invention will generally have a leaching agent concentrationwhich is greater than the concentration of leaching agent present in thesecond aqueous raffinate, the second aqueous raffinate produced inaccordance with the processes of the present invention will generallyhave a leaching agent concentration which is greater than theconcentration of leaching agent present in the third aqueous raffinate,and so on, with the aqueous raffinate from a solvent extractioncomponent of any additional “intermediate” sub-circuit in accordancewith the processes of the present invention generally having a leachingagent concentration which is greater than the concentration of leachingagent present in the aqueous raffinate from the solvent extractionvessel next following in the instant process. In preferred embodimentsof the present invention, the first aqueous raffinate will have aleaching agent concentration which is at least 10% greater than theconcentration of leaching agent present in the second aqueous raffinate,while in increasingly more preferred embodiments of the presentinvention, the first aqueous raffinate will have a leaching agentconcentration which is at least 20% greater, preferably at least 50%greater, more preferably at least 75% greater and most preferably 100%greater. Such differentials of concentrations of leaching agents insecond aqueous raffinates over the concentration of leaching agent inthe third aqueous raffinate are similar to those between the first andsecond aqueous raffinates. The concentration of leaching agent in anyintermediate aqueous raffinate over the concentration of leaching agentis the aqueous raffinate from the next-following solvent extractionvessel in the present process are also similar to those between thefirst and second aqueous raffinates. The aqueous stream for diluting thefourth aqueous leach pulp (step (g)) is normally the raffinate from thefinal solvent extraction process, optionally with neutralization oroptionally following metal recovery of a second valuable metal value,but it may comprise fresh water introduced into the process and/or aportion of other aqueous process streams to maintain a water balance.Where the leaching agent comprises an acid, any of the aqueousraffinate(s) may be at least partly neutralized (e.g., to any pH up toabout 8) with any basic substance (e.g., lime when the leaching agent issulphuric acid) prior to its use for diluting the fourth aqueous leachpulp in the third solid-liquid separation.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the description of the process hereinand the Claims attached hereto.

A computer simulation, based on mass balance principles and usingiterative Excel spreadsheets, was run to compare the economics of theconventional leaching and solvent extraction circuits in widespread use(illustrated in FIG. 1), against the economics of the split circuitconfiguration of the leaching and solvent extraction stages currently inuse in some plants (illustrated in FIG. 2), and against the economics ofthe simplest configuration of the instant invention (illustrated in FIG.3). In this non-limiting simulation, copper was used as the desiredmetal to be recovered, sulphuric acid was used as the leaching agent,and all numbers expressing quantities or concentrations are to beunderstood as approximations (to be understood, where not alreadypresent, as modified by “about”), not representations, for comparisonpurposes only, and affected by the ore grade, the water content of thecrushed ore solids, the metal recovery achieved in leaching, the desiredpulp density in leaching, desired wash ratios and thickener flowdensities achievable in a CCD solid-liquid separation, the response ofthe leached solids to solid-liquid separation, the total flow of leachsolution to be treated, the design of the solvent extraction process,and other parameters determined by the plant operators. The simulationis intended to be illustrative of the instant invention's advantages,but should not be interpreted to limit the scope of the currentinvention in any way. For purposes of the simulation, the followingconditions were used:

TABLE 1 Case Study Basis Ore treated (tons per day) 8,700 Ore grade (%Cu) 3.5 Ore Specific Gravity 2.8 Pre-leach thickener U/F (% solids) 55 %Solids in Leach 22 % Recovery in Leach 90 All CCD thickener U/F (%solids) 50 Number of CCD stages 6 Wash ratio in CCD 2:1 Copper recoveryin each SX unit (%) 90

The economic benefits of the Sequential Circuit flow sheet relative toboth the Split Circuit flow sheet and the conventional flow sheet aredetailed in Table 2.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow diagram, containing pertinent components of the massbalance for the circuit, representing the flows in a standardconventional agitation leaching-solvent extraction flow sheet, whereinall of the aqueous leach solution is treated in the same manner.

FIG. 2 is a flow diagram, with pertinent mass balance values,representing a “Split Circuit” flow sheet, wherein an aqueous leachsolution is divided into two portions—one portion without significantdilution and the other with water dilution—prior to being subjected tosolvent extraction.

FIG. 3 is a flow diagram, with pertinent mass balance values,representing an embodiment of the “Sequential Circuit” flow sheetaccording to the present invention.

Optional units/operations are shown in dashed lines, with a cobaltrecovery unit representing one or more units for recovering other metalvalues that may be present in sufficient quantities in the incoming ore.

EXAMPLES Comparative Example A

Comparative Example A is based on FIG. 1, which depicts a process flowdiagram of a standard conventional copper agitation leaching and solventextraction circuit, with pertinent mass balance numbers included foraqueous flows, copper concentrations and acid concentrations.

The leach pulp exiting the leach unit/train (“LEACH”), consisting ofabout 1224 cubic meters/hour of aqueous leach solution, comprising 9.92g/l of copper and 2.0 g/l of sulphuric acid, and about 362.5 tonnes/hourof crushed and milled ore less the mass leached, is mixed/washed, in acounter-current decantation (“CCD”), with about 622 cubic meters/hour ofrecycled aqueous raffinate from the copper solvent extraction unit/train(“SX1”). For modelling purposes, the 622 cubic meters/hour of raffinatecontaining 0.80 g/l Cu was assumed to be neutralized to contain 2 g/lsulphuric acid before addition to the CCD circuit as wash solution, thusdiluting the copper concentration of the aqueous leach solution exitingthe CCD circuit from about 9.92 g/l copper to about 8.05 g/l copperprior to this solution being fed to the solvent extraction. An aqueousleach solution obtained from the CCD of about 1535 cubic meters/hour,comprising 8.05 g/l copper and 2.0 g/l sulphuric acid, is circulated toSX1, and an aqueous raffinate, comprising 0.80 g/l copper and 13.2 g/lsulphuric acid, exiting from the SX1, is split, with about 913 cubicmeters/hour being recycled back to the leaching operation, where theacid is used to dissolve more copper, and about 622 cubic meters/hourthat is recycled to neutralization and then to the CCD. The about 622cubic meters/hour of raffinate, which is recycled to the CCD operation,is used to wash the leach solution from the leached solids, in order tominimize soluble metal losses in the aqueous phase portion of theleached pulp that is eventually disposed to tailings. A small portion offresh water may be added to the overall leach/wash system or a smallportion of aqueous solution may be bled from the overall leach/washsystem to maintain a water balance.

In Comparative Example A, 913 cubic meters per hour of raffinatecontaining 13.2 g/l sulphuric acid would be returned to leaching, about622 cubic meters per hour of raffinate containing 13.2 g/l sulphuricacid is neutralized to 2.0 g/l sulphuric acid prior to recycle back tothe CCD circuit, and about 311 cubic meters per hour of aqueous solutioncontaining 0.91 g/l copper is lost in tailings.

Comparative Example B

Comparative Example B is based on FIG. 2, which depicts a process flowdiagram of a “Split Circuit” copper leaching and solvent extractionsystem, with pertinent mass balance numbers included for aqueous flows,copper concentrations and acid concentrations. The leach pulp exitingthe leach unit/train (“LEACH”), consisting of about 1224 cubicmeters/hour of aqueous leach solution, comprising 9.92 g/l of copper and2.0 g/l of sulphuric acid, and about 362.5 tonnes/hour of crushed andmilled ore, less the mass leached, is passed to an initial solids-liquidseparation (S/L), comprising a clarifier using decantation. Then about913 cubic meters/hour of this solution, containing about 10.07 g/l Cuand about 2.0 g/l sulphuric acid, is taken directly to solventextraction (SX 1), where the copper is extracted and sulphuric acid isregenerated. SX 1 will reasonably produce a raffinate containing about1.01 g/l copper and about 15.95 g/l sulphuric acid, which solution isthen recycled back to leaching. The leach pulp exiting the initialsolid-liquid separation, which contains about 311 cubic meters/hour ofleach solution, is taken to a counter-current decantation wash circuit(CCD) where it is mixed with about 622 cubic meters/hour of raffinatefrom SX 2 that has been, optionally, partially neutralized to 2.0 g/lsulphuric acid. About 622 cubic meters/hour of leach solution from theCCD circuit, comprising 5.21 g/l copper and 2.0 g/l sulphuric acid, istaken to SX 2 to give a raffinate containing 0.52 g/l copper and 9.2 g/lsulphuric acid. A small portion of fresh water may be added to theoverall leach/wash system or a small portion of aqueous solution may bebled from the overall leach/wash system to maintain a water balance.

In Comparative Example B, 913 cubic meters/hour of raffinate containing15.95 g/l sulphuric acid is returned to leaching, about 622 cubicmeters/hour of raffinate from SX 2 containing 9.2 g/l acid isneutralized to 2.0 g/l sulphuric acid and about 311 cubic meters perhour of aqueous solution containing 0.67 g/L copper is lost to Tails.

The amount of acid in any aqueous stream at a particular time is thestream flow at that time multiplied by the acid concentration in thestream. A simple calculation shows that for this particular case about2.51 more metric tons of acid/hour, or about 60.3 more metric tons ofacid/day, is recycled to leaching using the “Split Circuit” flow sheetof Comparative Example B over the standard conventional flow sheet ofComparative Example A. Acid costs vary widely from, currently, aboutUS$60/ton to above US$250/ton depending on the location. For low-costacid, the savings in acid using the “Split Circuit” flow sheet insteadof the conventional standard flow sheet would be about US$3618/day,while for high-cost acid, the savings in acid would be aboutUS$15,075/day or greater.

A second simple calculation shows that for these Comparative Examples,about 2.49 less metric tonnes of acid per hour, or about 59.8 lessmetric tonnes acid per day, are neutralized using the “Split Circuit” ofComparative Example B over the standard conventional circuit ofComparative Example A. This is well within rounding error, since thegreater amount of acid recycled to leaching using the “Split Circuit”flow sheet of Comparative Example B, over the conventional circuit flowsheet of Comparative Example A, should equal the lesser amount of acidthat is neutralized using the “Split Circuit” flow sheet of ComparativeExample B compared to the acid that is neutralized using theconventional circuit flow sheet of Comparative Example A.

Savings in neutralization can vary widely, depending on the cost of theneutralizing agent (typically lime or limestone), the capital requiredto build a larger neutralization plant and the cost to dispose of thegreater amount of gypsum formed.

The amount of copper in any aqueous stream at a particular time is thestream flow at that time multiplied by the copper concentration in thestream. A third simple calculation shows that the total copper recoveredusing the “Split Circuit” flow sheet of Comparative Example B is greaterthan the total copper recovered using the standard conventional flowsheet of Comparative Example A by 74.64 kilograms/hour or about 1.79tonnes/day. At a copper price of, currently, US$2.50/pound, thisadditional copper has a value of US$9,866/day or about US$3.55Mannually.

Example 1

Example 1 illustrating the present invention is based on FIG. 3, whichdepicts a process flow diagram of a simple example of a copper leachingand solvent extraction process according to the instant invention(denominated “Sequential Circuit”), with pertinent mass balance numbersincluded for aqueous flows, copper concentrations and acidconcentrations. In this Example 1, a leach pulp consisting of about 1224cubic meters/hour of aqueous leach solution, comprising 7.56 g/l ofcopper and 2.0 g/l of sulphuric acid, and about 362.5 tonnes/hour ofcrushed and milled ore less the mass leached, flows directly withoutdilution from this first leach unit (Leach 1), where about 75% of thecopper from the crushed, mined ore has been dissolved into an aqueousacidic leach solution, to a solid-liquid separator (S/L1). Fromseparator S/L1 about 913 cubic meters/hour of an aqueous leach solution,comprising 7.56 g/l copper and 2.0 g/l sulphuric acid, is circulated toa solvent extraction unit/train (SX1), and a pulp, consisting of about311 cubic meters/hour of leach solution and about 362.5 tonnes/hour ofpartially leached crushed and milled ore less the small mass of oreleached in Leach 1, is sent to a second leach unit. The entire 913 cubicmeters/hour of metal-depleted aqueous leach solution, comprising 12.5g/l sulphuric acid and 0.77 g/l copper, exiting SX1 is recycled to LEACH1 where the acid contained in this first raffinate is used to leachcopper from more fresh crushed and milled ore solids.

The remaining amount of copper in the partially leached crushed andmilled ore solids exiting Leach 1, (25% of the original amount in thecrushed and milled ore solids entering Leach 1) is then dissolved fromthe solids in LEACH 2, from which 1224 cubic meters/hour of aqueousleach solution, comprising 4.6 g/l copper and 2.0 g/l sulphuric acid,and about 362.5 tonnes/hour of solids less the total mass leached, issent to another solid-liquid separator (S/L2), again, without dilution.About 913 g/l of a second aqueous leach solution, comprising about 4.6g/l copper and 2.0 g/l sulphuric acid emerges from S/L2 and iscirculated to a second solvent extraction unit (SX2), and a pulp,consisting of about 331 cubic meters/hour of leach solution and about362.5 tonnes/hour of almost totally leached solids less the massleached, is sent to a third solid-liquid separator (CCD) where the pulpis diluted and washed The entire 913 cubic meters/hour of metal-depletedaqueous leach solution, comprising 8.4 g/l sulphuric acid and 0.46 g/lcopper, are recycled from SX2 to LEACH 2 where the acid contained inthis second raffinate is used to leach copper from the partially leachedcrushed and milled ore solids entering Leach 2 from S/L1.

Exiting the CCD wash process is a pulp, consisting of 311 cubicmeters/hour of aqueous solution, comprising 0.31 g/l copper and 2 g/lsulphuric acid, and 362.5 tonnes/hour of almost totally leached solids,which is sent to tails and 622 cubic meters/hour of a third aqueousleach solution, comprising 2.4 g/l copper and 2.0 g/l sulphuric acid,which is sent to a final solvent extraction unit (SX3). From SX3, 622cubic meters/hour of metal depleted aqueous leach solution, comprising0.24 g/l copper and 5.3 g/l sulphuric acid, emerge as an aqueousraffinate for possible neutralization (“Neut”) of excess acid beforepossible recovery of additional metal (“Co”), and recycle to the CCD aswash solution or supplemental wash solution.

In Example 1, about 913 cubic meters/hour of raffinate containing 12.5g/l acid and about 913 cubic meters/hour of raffinate containing 8.4 g/lacid are returned to Leach 1 and 2, respectively. About 622 cubicmeters/hour raffinate from SX 3 containing 5.3 g/l acid is neutralizedto 2 g/l acid and about 311 cubic meters/hour of aqueous solutioncontaining 0.31 g/l copper is lost to tailings

Example 3 Economic Benefit Calculation

TABLE 2 Economic Benefits of the Sequential Circuit and Split CircuitFlowsheets over the Conventional Flowsheet in a One Year Period of TimeConven- Split Sequential tional Circuit Circuit Operating days per year360 360 360 Neutralization Cost 200 200 200 ($/ton acid) Cu price ($/lb)2.50 2.50 2.50 Acid to neutralization 167.2 107.5 49.7 (MT/day)Neutralization cost 12.04 7.74 3.58 ($ million/yr) Benefit ($million/yr) 4.30 (A) 8.46 (A) Cu soluble loss (ton/day) 6.80 5.00 2.31Revenue loss ($ million/yr) 13.45 9.90 4.57 Benefit ($ million/yr) 3.55(B) 8.88 (B) Total benefit ($ million/yr) 7.85 (A + B) 17.34 (A + B)

Calculation of the financial savings the invention offers when comparedto the conventional flow sheet and the split circuit flow sheet arebased on three advantages of the invention: the total greater amount ofleaching agent recycled to Leach 1 and Leach 2, the decrease in acidneutralized and the decrease in the concentration of the desired metalin the aqueous solution leaving the circuit to the tailings with thewashed leach pulp exiting the CCD wash circuit (called the “coppersoluble loss”).

In Table 2, the neutralization cost/year is calculated by multiplyingthe acid neutralized/day by the cost of neutralization times 360days/year. The acid neutralized/day is the flow of the respective steambeing neutralized multiplied by the g/l acid neutralized, for example,the acid neutralized/day for the conventional circuit is [622 cubicmeters/hour times (13.2 g/l acid−2 g/l acid) times 24=167.2]. The costof neutralization, assumed to be US$200/tonne acid, is the total cost ofthe acid plus the cost of the base needed to neutralize the acid plus asmall operating cost. A neutralization cost of US$200 tonne acid is usedfor this example and such cost is reasonable for neutralization and wellwithin the range of today's costs for neutralization. The benefit of theacid savings on an annual basis for the Sequential Circuit flow sheetaccording to the instant invention over the Split Circuit flow sheet andover the conventional standard flow sheet is calculated from thedifference in the neutralization cost for each of the three flow sheets.

Also in Table 2, the benefit on an annual basis associated with thelower soluble copper loss offered by the by the Sequential Circuit flowsheet according to this instant invention over the “Split Circuit” flowsheet and over the conventional standard flow sheet is determined fromthe differences in the economic value of the soluble copper lost on anannual basis (the concentration of the copper in the respective streamsexiting the CCD wash circuit to Tails times the flow calculated on anannual basis times the copper price) for each of the three flow sheets.

From Table 2 it can be seen that the use of the Sequential Circuit flowsheet according to the instant invention offers an annual savings ofUS$17.34 million over the conventional leaching-solvent extraction flowsheet and a savings of US$9.49 million over the use of the Split Circuitflow sheet (which Split Circuit flow sheet has shown an annual savingsUS$7.85 million over the conventional flow sheet).

Clearly, the use of the process according to the present invention wouldresult in much more leaching agent (in these Examples, acid) beingreturned to additional leaching than would be recycled with theconventional standard flow sheet or with the split circuit flow sheet.Also clearly the use of the process according to the instant inventionwould result in more copper being produced, and less copper lost assoluble copper, when compared to the conventional standard flow sheetand the split circuit flow sheet.

Since the amount of ore and copper content of the ore being treated isthe same for the Conventional Circuit (Comparative Example A), the SplitCircuit (Comparative Example B) and the Sequential Circuit according tothis invention (Example 1), a direct and valid comparison can be madefor the amount of acid neutralized and the soluble copper loss usingeach flow sheet.

The values calculated in Table 2 are both realistic and reasonableconsidering that, in December, 2007; the price of acid varies betweenUS$60/tonne to over 250/tonne, depending on location and logistics, withmost acid prices well above the low figure of US$60/tonne. Also at thepresent time the price of copper is about US$3.00/pound.

No matter what values are used to calculate the annual total benefit inUS$, some benefit in less acid neutralized and more copper produced isalways present for the Sequential Circuit flow sheet according to thisinstant invention over the Split Circuit flow sheet and the conventionalstandard flow sheet.

In addition to the benefits of more acid recycled to leaching, less acidbeing neutralized and less copper being lost as soluble copper in thepulp exiting the CCD process, a fourth advantage of the SequentialCircuit flow sheet pertains to the leaching efficiency. When theleaching process is divided into a first leach, where a majority of thecopper is leached, and a second leach, where the remainder of the copperis leached, the copper concentration in the aqueous phase in contactwith the almost-totally-leached, crushed and milled ore solids in thelast leach unit is considerably lower (4.60 g/l Cu) than when all theleaching is done in one leaching unit/train (10.07 g/l Cu in the case ofthe split circuit flow sheet and 9.92 g/l Cu in the case of the standardconventional flow sheet). A lower copper concentration in the leachsolution in contact with the final leached solids should allow a veryslightly higher overall leach recovery because the diffusion of leachedcopper from the pores in the ore particles is faster. In addition, theacid to leach can be better controlled and thus made more efficient.

A fifth benefit may occur in those cases where the acid in the streambeing recycled to the CCD wash process does not need to be neutralized,but the bleed of this stream from which a component of value in thebleed is recovered, for example cobalt, must be neutralized prior tocobalt recovery. Neutralization with a soluble base, such as caustic orammonia, is very expensive, thus the lower the acid content of the bleedstream, the lower the amount of expensive base needed forneutralization. Furthermore, the use of a solution of caustic forneutralization adds water to the bleed stream, thereby diluting thevaluable cobalt stream. Alternatively, neutralization can take placewith lime or limestone, which is a less costly base. In this case, alesser amount of acid in the bleed stream requires less lime orlimestone for neutralization, and in the process, a lesser amount ofgypsum precipitate, that must be removed from the system, is produced. Alesser amount of gypsum allows the use of smaller equipment for thisparticular solid-liquid separation. Since, when finely-divided solidsseparated from a liquid, the solids will always contain some of theliquid, a lesser amount of gypsum will contain a lower volume of theneutralized bleed stream that contains the valuable second component,for example, cobalt. Thus, the ultimate recovery of the secondaryvaluable component in the bleed stream is higher when using the processaccording to the invention.

1. A process for recovering desired metal values from crushed and milledore solids comprising the steps of: (a) mixing a first aqueous leachsolution with the crushed and milled ore solids in a first agitated tankleach unit, whereby at least a portion of the desired metal values inthe ore solids is dissolved into the first aqueous leach solution toobtain a first aqueous leach pulp comprising a mixture of leached solidsand first aqueous leach solution; (b) subjecting the first aqueous leachpulp to a first solids-liquid separation, without significant waterdilution, to provide a first clarified aqueous leach solution and asecond aqueous leach pulp, wherein the second aqueous leach pulpcomprises leached solids at a percent solids level that is greater thanthat in the first aqueous leach pulp; (c) subjecting the first clarifiedaqueous leach solution to a first solvent extraction, whereby at least aportion of the desired metal values are extracted into a first organicphase comprising one or more extraction reagents specific for thedesired metal, and a first aqueous raffinate, depleted of desired metalvalues, is obtained; (d) mixing a second aqueous leach solution with thesecond aqueous leach pulp in a final agitated tank leach unit, wherebyat least a portion of the desired metal values formerly in the secondaqueous leach pulp is dissolved into the second aqueous leach solutionto obtain a third aqueous leach pulp, wherein the third aqueous leachpulp comprises a mixture of twice-leached solids and a second aqueousleach solution, rich in desired metal values; (e) subjecting the thirdaqueous leach pulp to a second solids-liquid separation, withoutsignificant water dilution, to provide a second clarified aqueous leachsolution and a fourth aqueous leach pulp, wherein the fourth aqueousleach pulp comprises leached solids at a percent solids level that isgreater than that in the third aqueous leach pulp; (f) subjecting thesecond clarified aqueous leach solution to a second solvent extraction,whereby at least a portion of the desired metal values are extractedinto a second organic phase comprising one or more extraction reagent(s)specific for the desired metal, and a second aqueous raffinate, depletedof desired metal values, is obtained; (g) subjecting the fourth aqueousleach pulp to a third solids-liquid separation, with significantdilution via an aqueous stream, to provide a third clarified aqueousleach solution and a fifth aqueous pulp, wherein the concentration ofdesired metal values in the third clarified aqueous leach solution isless than the concentration of desired metal values in the secondclarified aqueous leach solution, and the fifth aqueous pulp comprises amixture of leached solids and aqueous leach solution; and (h) subjectingthe third clarified aqueous leach solution to a third solvent extractionwhereby at least a portion of the desired metal values are extractedinto a third organic phase comprising one or more extraction reagents(s)specific for the desired metal, and a third aqueous raffinate, depletedof desired metal values, is obtained.
 2. The process according to claim1, wherein the providing of a second aqueous leach pulp in step (b)further comprises an intermediate leaching step, wherein a firstintermediate aqueous leach pulp obtained from the first solid-liquidseparation is mixed with an intermediate aqueous leach solution in anintermediate agitated tank leach unit to obtain a second intermediateaqueous leach pulp, subjecting the second intermediate aqueous leachpulp to an intermediate solid-liquid separation to obtain the secondaqueous leach pulp and an intermediate clarified aqueous leach solution,wherein the intermediate clarified aqueous leach solution is subjectedto an intermediate solvent extraction to obtain an intermediate aqueousraffinate.
 3. The process according to claim 1, wherein the desiredmetal is selected from the group consisting of copper, zinc, nickel andcobalt.
 4. The process according to claim 1, wherein the first aqueousleach solution and the second aqueous leach solution comprise sulphuricacid.
 5. The process according to claim 1, wherein the first aqueousleach solution and the second aqueous leach solution comprise ammonia.6-9. (canceled)
 10. The process according to claim 1, wherein the thirdsolid-liquid separation comprises counter-current decantation.
 11. Theprocess according to claim 1, wherein the concentration of the desiredmetal in the first clarified aqueous leach solution is at least 30%greater than the concentration of the desired metal in the secondclarified aqueous leach solution.
 12. The process according to claim 1,wherein the concentration of the desired metal in the first clarifiedaqueous leach solution is at least 50% greater than the concentration ofthe desired metal in the second clarified aqueous leach solution. 13.The process according to claim 1, wherein the concentration of thedesired metal in the first clarified aqueous leach solution is at least70% greater than the concentration of the desired metal in the secondclarified aqueous leach solution.
 14. The process according to claim 1,wherein the concentration of the metal in the first clarified aqueousleach solution is at least 100% greater than the concentration of thedesired metal in the second clarified aqueous leach solution.
 15. Theprocess according to claim 1, wherein the first aqueous leach solutioncomprises at least a portion of the first aqueous raffinate.
 16. Theprocess according to claim 15, wherein the first aqueous leach solutionfurther comprises fresh leaching agent.
 17. The process according toclaim 16, wherein the second aqueous leach solution comprises at least aportion of the second aqueous raffinate.
 18. The process according toclaim 17, wherein the second aqueous leach solution further comprisesfresh leaching agent.
 19. The process according to claim 18, wherein thefirst aqueous leach solution, second aqueous leach solution, and freshleaching agent comprise sulphuric acid.
 20. The process according toclaim 18, wherein the first aqueous leach solution and second aqueousleach solution comprise ammonia, and the fresh leaching agent comprisesgaseous ammonia and/or ammonium hydroxide.
 21. The process according toclaim 1, wherein the third aqueous raffinate is neutralized and furtherprocessed according to a step selected from the group consisting ofbeing circulated back to the third solids-liquid separation, being sentto disposal, being treated to recover one or more other metal values,and combinations of two or more of these.